The present invention relates to a process for producing pulp molded articles. More particularly, it relates to a process for producing pulp molded articles which is suitable to production of hollow molded articles having an opening whose cross-sectional area is smaller than that of their bodies.
Pulp molded articles have been produced by a process in which a pair of split molds comprising a male mold and a female mold are used and pulp slurry is charged between the pair of split molds followed by dehydrating the pulp slurry to form shallow trays, plates, etc. or a process in which a pair of split molds are used to mold two split molds which are combined together to form a hollow molded article such as a bottle.
In the process using a male mold and a female mold, it is not easy to produce molded articles of complicated shape such as undercut containers. In the process comprising combining split parts, the butt-seam which appears on the surface impairs the strength and appearance of molded articles, and it is not easy to produce hollow molded articles having a smaller cross-sectional area at the opening than that in the body.
Separately, production of pulp molded articles further includes a step of dehydrating the water-containing pulp layer formed by a paper making technique for improving handling properties and for reducing the drying time. Known methods of dehydration include a method comprising pressing a pulp layer by use of an elastic member as described in Japanese Patent Application Laid-Open No. 156600/93 and a method comprising pressing a pulp layer by use of a flexible film as disclosed in Japanese Patent Application Laid-Open No. 223230/95.
In these methods of dehydration, when the pressing force is increased in order to decrease the water content of a wet pulp layer, a paper making net bites the pulp layer to leave its trace on the surface of the pulp molded article, which deteriorates the appearance of the molded article. Besides, a large-sized apparatus is required to increase the pressing force. Further, since the degree of dehydration achieved by mechanical manipulation is limited, a long time would be required to reduce the water content to a satisfactory level, showing poor dehydration efficiency.
The pulp layer having been dehydrated so as to have a prescribed water content (hereinafter referred to as a preform) is removed from the paper making mold and dried by heating in a heating mold. The lower the water content of the preform, the shorter the required drying time in the heating mold.
However, in case where a preform is to be shaped while dried in a heating mold, the shape of the inner wall of the mold sometimes fails to be precisely transferred to a preform with too low a water content due to insufficient mobility of the pulp fiber. In particular, it is difficult to transfer the shape of threads or impressions for making letters, figures, symbols, etc. in relief.
Accordingly, an object of the present invention is to provide a process for producing pulp molded articles of complicated shape and free from a butt-seam.
Another object of the invention is to provide a process for easily producing hollow pulp molded articles having a smaller cross-sectional area at the opening than that in the body.
Still another object of the invention is to provide a process for producing pulp molded articles in which a pulp layer can be dehydrated at high efficiency with a simple apparatus without leaving the trace of a paper making net on the surface of the molded articles.
Yet another object of the invention is to provide a process for producing pulp molded articles in which a water-containing preform can be dried at improved efficiency while the shape of the inner wall of a mold can be transferred with satisfactory precision.
The above objects are accomplished by a process for producing a pulp molded article having an opening which comprises forming a pulp layer on the inner wall of a mold cavity and drying the pulp layer, wherein a hollow inflatable pressing member which has been made small so as to have smaller cross-sectional contour than that of the portion of the pulp layer corresponding to the opening of the pulp molded article is inserted into the mold cavity after the pulp layer is formed, and a pressurizing fluid is fed to the inside of the inserted pressing member to inflate the pressing member thereby to press the pulp layer onto the inner wall of the cavity.